Semiconductor devices are continuously improved to enhance device performance. A fundamental building block of semiconductor devices is, for example, a transistor device which includes a gate stack on a semiconductor substrate. The gate stack includes a gate electrode over a gate dielectric layer. Smaller devices and shrinking ground rules are the keys to enhance performance and to reduce cost. As devices are being scaled down, the technology becomes more complex, and changes in device structures and materials as well as new fabrication methods are needed to maintain the expected performance enhancement from one generation of devices to the next.
High-K metal gate transistors have been investigated for future devices. A high-K metal gate transistor includes a metal gate electrode layer over a high k gate dielectric layer. However, high-K metal gate transistors exhibit high VT roll up for narrow width devices, in particular for n-type devices. High VT roll up is undesirable as this degrades the narrow device Ieff and performance, where Ieff refers to the on current of the device.
From the foregoing discussion, it is desirable to provide a high-k metal gate transistor with improved performance and reliability.